B-15A, B-15J, B-15K, and C-16 icebergs in the Ross Sea, Antarctica

Credit:

Jacques Descloitres, MODIS Rapid Response Team, NASA/GSFC

Summer’s thaw normally releases Antarctica’s Ross Sea and McMurdo Sound from the thick sea ice that accumulates over the winter, but in 2005, the process was blocked. The massive B-15A iceberg is disrupting the normal wind and current patterns that break up the ice, leaving McMurdo Sound frozen.

McMurdo Sound passes through an annual cycle in which thick ice freezes on the water during Antarctica’s frigid winter, then breaks and drifts into the Ross Sea during the summer. By late spring in early November, a channel of ice has typically been swept from the Sound. During the spring and summer of 2004-2005, the process was disrupted by the giant B-15A iceberg. Topping 129 kilometers (80 miles) in length, the Long-Island-sized iceberg blocked the currents that usually clear out the Sound. The ice did not show signs of breaking up until early January 2005.

In early January, temperatures heated up and a powerful storm moved over Antarctica. Strong winds churned the ocean, and, along with warmer temperatures, may have contributed to the break-up of the ice. When the clouds cleared on January 13, the formerly solid ice had been broken into chunks.

The effect of the B-15A iceberg on McMurdo Sound is apparent in this series of true-color images, acquired by the Moderate Resolution Imaging Spectroradiometer (MODIS). In these images, the long B-15A iceberg drifts away from Ross Island on the open waters of the Ross Sea. The B-15J iceberg, which broke from B-15A in October 2003, rotates and also begins to drift into the sea. The frozen McMurdo Sound is a bluish white to the left of the two large icebergs. To their right is the smooth white Ross Ice Shelf, the large sheet of floating ice from which the B-15 iceberg calved in 2000. The rugged fjords of Scott Coast are partially free of snow in the bottom center of the images.

The frozen expanse of water between the shore (left) and the open sea (right) had been a serious problem for penguins, which had to travel a greater distance to reach open waters and food. Though adults were probably able to make the trip to feed themselves, scientists feared the adults would have to consume most of the food they were bringing to their chicks because of the increased length return journey. When the ice broke in January, the trip became shorter and may have brought some relief to the penguins. It is not clear if the changing conditions came in time to save the penguin chicks from starvation.

The ice was also a potential problem for supply ships trying to reach McMurdo Station and Scott Base. Instead of cutting through the usual 64 kilometers (40 miles) that separate the pier from open waters, icebreakers had to chart a course through 129 kilometers (80 miles) of ice. The icebreaker succeeded in reaching McMurdo in early January.

This series of images also tracks the movement of the B-15A iceberg towards the Drygalski Ice Tongue. For much of December and early January, the massive iceberg had been on course to strike the ice tongue in what could have been a collision of giants. The Drygalski Ice Tongue is a floating extension of a land-based glacier, and can be seen on the left edge of the images as the long, smooth white barrier separating the open sea from the frozen McMurdo Sound.

Such ice tongues have been known to break under smaller strains, and according to NASA scientist Robert Bindschadler, the Drygalski Ice Tongue has never experienced a blow of the magnitude that B-15A could deliver. The iceberg had been moving steadily towards the ice tongue, but its movement slowed in late December. Just as the gap between the two narrowed to less than 4 kilometers, the iceberg rotated slightly and may have become grounded. By January 13, the gap had widened as the iceberg appeared to reverse its course, perhaps in response to being grounded. It was late March before the iceberg began to move again, its tip just missing the Drygalski Ice Tongue as it started to drift into the Ross Sea.

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